Thermostatic heater device for liquids, in particular for the water of aquariums

ABSTRACT

A thermostatic heater device for liquids, in particular for the water of aquariums, comprising within a tubular casing: a rigid support ( 2 ) constructed of electrically insulating, heat-conducting material, at least one layer of electrically insulating, heat-resistant flexible film ( 14 ) wound about said tubular support, at least one resistive element ( 16 ) arranged on a dielectric film wrapped about said first flexible film, a layer of a second electrically insulating, heat-resistant flexible film ( 14 ) wound about said dielectric film already wrapped about said layer of first flexible film, means ( 22 ) for connecting said resistive element to the electrical supply, and hydraulic sealing means ( 4, 6 ) between said tubular support ( 2 ) and said casing ( 8 ).

This invention relates to a thermostatic heater device for liquids, inparticular for the water of aquariums.

Heater devices with temperature control for liquids, in particular forthe water of aquariums, are known. One of these is described in U.S.Pat. No. 5,750,960 and comprises a dielectric film on which there isprinted a resistive element electrically powered via a thermostat.

This dielectric film is housed within an annular interspace bounded bytwo cylindrical walls, the inner of which is constructed of glass andthrough the interior of which the liquid to be heated passes. Oneportion of the annular interspace has a greater width to define achamber housing the thermostat, which is provided with a heat-sensitiveelement facing the heating element in such a manner as to determine itstemperature and hence, indirectly, the temperature of the watercirculating through the device.

This known heater device has proved well suited in particular tomaintaining the water of an aquarium at the desired temperature, but atthe same time has highlighted some aspects which could be improved.

One of these is that the heating element is in direct contact with theglass of the inner cylindrical wall, and hence does not posses thecharacteristic of double electrical insulation, as currently prescribedfor all non-earthed electrical appliances not powered at low voltage. Inthis respect, the provision of double insulation poses constructionalproblems which up to the present time have not been satisfactorilysolved.

A further improvable aspect is that in that particular device theheat-sensitive element is positioned to the side of the heating element,not in contact with the cylindrical glass wall. Because of this thesensitivity of the regulation is influenced by the temperature not onlyof the water, ie the quantity effectively to be controlled, but also ofthe heating element.

An object of the invention is to provide a thermostatic heater device inwhich, in contrast to known devices, those parts under voltage areprovided with double insulation, so satisfying current regulations.

A further object of the invention is to provide a thermostatic heaterdevice which is of extremely simple production and safe and reliableoperation, even if abnormal situations arise.

These and further objects are attained according to the inventionthrough a thermostatic heater device as claimed in claim 1.

A preferred embodiment of the invention is described in detailhereinafter with reference to the accompanying drawings, on which:

FIG. 1 is a schematic longitudinal section through a thermostatic heaterdevice according to the invention,

FIG. 2 is a schematic plan view of a resistive element obtained fromsilk-screen printed film,

FIG. 3 is a schematic view of certain successive stages of the procedurefor constructing the device, and

FIG. 4 is an exploded perspective view of a heater device of theinvention as used to form a thermal pump for an aquarium.

As can be seen from the figures the device of the invention comprises atubular support 2 constructed of electrically insulating,heat-conducting rigid material about which a heating element, indicatedoverall by 12, is wound.

The tubular support preferably consists of a piece of cylindrical glasstube open at its ends, on which there are mounted two preferably rubberannular gaskets, namely a lower one 4 and an upper one 6. Both thegaskets have an annular seat to house the corresponding end of the glasssupport 2 and have their outer annular surface provided with a pluralityof parallel circumferential ribs arranged to form a substantial sealwith a cylindrical casing 8.

The upper gasket 6 differs from the lower gasket 4 by the presence of anappendix 10 comprising an axial sealing passage for the electricalconnections of the heating element.

The heating element 12 preferably consists of a mylar film 14 on which aresistive ink is printed by traditional silk-screen printing. FIG. 2shows a plan view of this heating element, which consists of arectangular mylar support of length just less than the circumference ofthe tubular support 2 and of height less than the distance between thetwo gaskets 4 and 6 applied thereto. It is provided with a PTC resistiveink deposit which extends over the entire surface of the mylar supportwith the exclusion of a small region to which there is glued atemperature sensor 18, which is to be located at the lower end of thedevice when installed. The resistive element and the circuit connectedto the temperature sensor extend beyond the rectangular mylar supportalong an appendix 20 forming part of the support and extending in thedirection away from the temperature sensor.

The resistive element can be advantageously obtained starting from amylar web on which the entire resistive circuit and the connections tothe outside is silk-screen printed with resistive ink, after which iscut by die-cutting to form at each blow two resistive elements withtheir respective appendices 20. The heat-sensitive element 18 is thenglued to each resistive element in the required position, so that ismakes electrical connection with the two tracks 22 of the controlcircuit.

The device of the invention is formed by winding a film of polyestermaterial on the tubular support 2 until a layer of the desired thicknesscompatible with insulation requirements has been formed on the outsideof said support, after which the heating element 12 already providedwith the temperature sensor is positioned on said polyester film and thewinding continued until the sensor is finally interposed between twolayers of insulating material. When winding is complete, the lowergasket 4 is applied to the tubular support followed by the upper gasket6, through which the appendix 20 with the electrical connections ispassed.

Hence in practice the heating element 12 of the device of the inventioncomprises a tubular support 2 with the gaskets 4, 6 applied to its ends,a first layer of polyester film wound on the tubular support, theresistive film adhering to the outside of the first layer and passingwith its appendix 20 through the upper gasket 6, and a second layer ofpolyester film covering the resistive film.

The heating element 12 formed in this manner can then be inserted intothe casing 8 of a traditional pump 24, which can be connected to theelectrical power and control components via the tracks of the appendix20.

A traditional thermal protector element (not shown on the drawings) inthe form of a thermal fuse or a bimetallic strip is advantageouslyprovided in correspondence with that edge of the resistive film distantfrom the temperature sensor, ie in the upper part of the heater device,to protect the device from excessive temperature increase.

The thermostatic heater device of the invention is particularlyadvantageous, and in particular:

it has double electrical insulation and hence satisfies the most recentregulations,

it uses a heat-sensitive element which is located much closer to thewater to be temperature-controlled than to the resistive element, and ishence substantially uninfluenced by factors which could alter itssensitivity. Moreover this heat-sensitive element is positioned incorrespondence with that end of the tubular support which is the firstto receive the flow of water to be controlled, with the result that thethermostatic heater cuts out very rapidly,

it comprises a thermal protector element positioned in the highest partof the heater device, ie in that part which is at the highesttemperature when no water is circulating. Consequently it is able to actrapidly in the case of accidental stoppage of the water circulationpump, to prevent negative consequences arising,

it achieves all the aforesaid advantages by means of a very simple andeconomical construction, together with considerable ease of assembly.

What is claimed is:
 1. A method for producing a thermostatic heatercomprising the steps of: continuously winding for at least twice anentire circumference of a tubular rigid hollow support constructed ofelectrically insulating heat-conducting material with an electricallyinsulating heat-resistant flexible film, introducing between a firstlayer and a second layer a portion of a dielectric flexible film havinga length corresponding to said circumference of said support, saiddielectric flexible film being provided with a resistive elementarranged on a surface of said film facing the first layer of saidelectrically insulating flexible film, providing said resistive elementwith connecting means to an electrical supply, introducing said supportwithin a tubular casing, and applying hydraulic sealing means betweensaid tubular support and said tubular casing.
 2. A thermostatic heaterdevice for liquids comprising within a tubular casing: a rigid tubularsupport constructed of electrically insulating, heat-conductingmaterial, at least one first layer of an electrically insulatingheat-resistant flexible film wound about said tubular support, at leastone resistive element arranged on a dielectric film wrapped about saidfirst flexible film, a second layer of said electrically insulating,heat-resistant flexible film wound about said dielectric film alreadywrapped about said first layer of flexible film, means for connectingsaid resistive element to the electrical supply, and hydraulic sealingmeans between said tubular support and said casing.
 3. A device asclaimed in claim 2, further comprising a thermostat acting on saidelectrical supply.
 4. A device as claimed in claim 2, wherein saidtubular support is constructed of glass.
 5. A device as claimed in claim2, wherein seal gaskets are mounted on ends of said support for itsinsertion into said casing.
 6. A device as claimed in claim 2, whereinan assemblage formed by said two film layers with said interposedresistive element extends into an appendix provided with conductingtracks.
 7. A device as claimed in claim 2, further comprising a thermalprotection element.
 8. A device as claimed in claim 2, wherein saidflexible material films are formed of polyester.
 9. A device as claimedin claim 2, wherein said resistive element is a layer of PTC ink.